Both Helmet Mounted Displays (HMD) and Head-Up Displays (HUD) typically use green light to display imagery. An aircrew helmet may have two visors; a HMD display visor which is a clear visor onto which the HMD projection apparatus projects an image; and a glare visor for providing a reduction in ambient brightness. The latter is deployed when the outside world is too bright for comfortable viewing.
When the two visors are used together the HMD display visor suppresses the brightness of the HUD display light as perceived by the helmet wearer. This occurs because HMDs display visors selectively reflect HMD display light (e.g. green light) to make a projected HMD image bright against the back-drop outside world view. But, if the HMD display visor is used with a HUD apparatus, which would typically generate projected images also in green light, the optical properties of the HMD display visor will attenuate the brightness of the HUD display light as seen by the helmet wearer.
If this conflict occurs when the ambient outside world view is of low to medium brightness, then the problem may be solved by increasing the HUD image brightness. However, this comes at the cost of greater HUD power consumption and reduced equipment lifetime. Furthermore, the brightness of both the HUD image and the HMD image are constrained by many factors such that achieving sufficient such extra brightness in the perceived HUD image, when both HUD and HMD images are to be viewed together, against a bright outside world, is very difficult.
The invention aims to provide an improved glare visor which may assist in enhancing HUD image contrast either when used in combination with a HMD display visor or when used in isolation.